Protection of drill pipe



' May 17, 1966 P, wlNG 3,251,427

PROTECTION OF DRILL PIPE Filed 001;. 2, 1963 FIG. I

S0011 P. Ewing INVENTOR.

ATTORNEY United States Patent 3,251,427 PROTECTION OF DRILL PIPE ScottPreston Ewing, Tulsa, Okla., assignor, by mesne assignments, to EssoProduction Research Company, a corporation of Delaware Filed Oct. 2,1963, Ser. No. 313,220 6 Claims. ((31. 175-320) This invention concernsthe protection of drill pipe. It is especially concerned with providingcathodic protection to drill pipe.

In the art of drillings wells for the production of oil and gas, themost commonly used method is the socalled rotary drilling method. In therotary drilling method, a drill bit is spended at the lower end of astring of drill pipe which is supported from the surface of the earth. Adrilling fluid is forced down through the drill string, through thedrill bit, back up to the surface through the annulus between the drillpipe and the Wall of the borehole. While the drilling fluid servesprimarily to carry the rock cuttings from the drill bit to the surface,it also serves to lubricate and cool the drill bit. The drill bitobtains its rotary motion from the drill pipe which is rotated from thesurface. The rotary drilling system has been used very extensively forthe last thirty or forty years; although it was used even before that.

The drill strings or pipes are subjected to very severe operatingconditions. The length of the drill string, of course, is that requiredto drill the hole to the depth desired, which in many instances is up to10,000 to 15,000 feet or deeper. The drill pipe, itself, is normallyabout 4 /2 inches in diameter. -In some instances it is desired to applyforces as high as 200,000 pounds or more to the bit in order to obtainoptimum drilling conditions. This is obtained by many methods such as,for example, the addition of heavy sections of drill pipe commonlycalled drill collars, positioned near the bit. In deep holes, a hightorsional stress is required to rotate the pipe and bit at high speed.The bendingstresses are also very high where the direction of the holechanges or deviates as usually occurs during normal drilling. This allincreases the stress and strain placed on the drill string. In'additionto the extreme stresses placed on a string of drill pipe, the drillpipe, itself, frequently operates in rather corrosive environments. Thecorrosive environment may be due to several causes such as, for example,salt water, carbon dioxide and hydrogen sulfide encountered in drillingthe well, which becomes mixed with the drilling fluid} Oxygen, whichdissolves in the drilling fluid while it is incontact with air, alsocontributes to the corrosiveness.

Failure or breakage of the drill pipe is a major problem which has longplagued the industry. Over the years and throughout most of the periodof extensive use of rotary drilling systems, much elfort has beendevoted toward reducing the failure of the drill pipe. One group ofdrill pipe failures is due to improper design, improper make-up ofjoints, faulty welding, excessive wear, and excessive stresses. Thesecauses are fairly well understood now and can be largely prevented byproper design and construction and use of the pipe. of drill pipefailures, and one of the biggest cause of failure, is corrosion fatigue.Corrosion fatigue of drill pipe is usually characterized by numeroussmall cracks near the point where the break in the drill pipe occurs.When corrosion occurs, rust tubercles form. The pits under the tuberclescontinue to penetrate which causes high stress concentrations. 'Thesehigh stress concentrations start corrosion fatigue cracks. The cracksare normally across the pipe and the breaks are transverse with littlenecking or ductility. This indicates that the A second group stressescausing the break are longitudinal, that is, they are usually caused byalternate bending of the pipe.

Various corrosion inhibitors have been added to the drilling fluid in anattempt to reduce the corrosion fatigue of the drill pipe. Althoughinhibitors have been found to be helpful, they have limitations and theyincrease drilling costs. One definite limitation is in drilling withair. Drilling with :air has become increasingly popular during the lastten years. In this method, air is the primary fluid that is circulateddown through the drill pipe for removing the cuttings up through theannulus. If the hole is relatively dry, the gas stream moves the chipsout of the hole and corrosion or corrosion fatigue is not normallyconsidered to be a serious problem in this situation. If a considerableamount of water flows into the borehole from penetrated water-bearingsands, the present trends is to remove the water with foam. In thissystem, the produced water is thoroughly aerated and it may in manyareas also contain hydrogen sulfide and/or carbon dioxide, all of whichtend to cause corrosion of the external surface of the drill pipe. Indrilling with foam, the amount of water flowing inside the drill pipe isrelatively small. It therefore requires only a small amount of anelfective inhibitor in the foam to eliminate or greatly reduce thecorrosion inside the drill pipe. However, it is not'economical toattempt to inhibit the large volume of produced water on the outside ofthe drill pipe. It is thus seen that there is a need, which has existedfor many years, for an inexpensive and effective way of preventingexternal corrosion of drill pipe. The present invention discloses such asystem. In accordance with the present invention, the drill pipe iscathodically protected, preferably by means of zinc ring members cast inintimate contact with the external surface of the drill pipe.

Various objects and a better understanding of the invention can be hadfrom the following description taken in conjunction with the drawing inwhich:

FIGURE 1 is a view of a string of drill pipe suspended in. the boreholein which the pipe has cathodic protection; and,

FIGURE 2 is a section view taken along the line 2- of FIGURE 1'.

Turning now to the drawing, there is illustrated a string of drill pipe10 suspended in a well bore 12. The string of drill pipe 10 is normallymade of high strength steel containing about 0.35 percent carbon andsmall amounts of alloying elements as chromium, nickel, molybdenum, etc.The string 10 is suspended from the surface by conventional equipmentnot shown and which also rotates the drill string. At the lower end ofthe drill string 10 is a bit 14. In the drill string 10 just about thebit 14 is a heavy section of drill pipe 16 which is commonly called adrill collar. The drill string 10 is made up of a large number ofsections or joints of drill pipe which are normally about 30 feet long.Shown in FIGURE 1 is an upper section or joint 18 and a lower joint ofdrill pipe 20. Drill pipe joint 20 is connected to the section of pipe18 by a tool joint 21 which comprises a box joint 22 and a pin joint 24all in a conventional manner. It will be noted that the joint 21 is of agreater diameter and therefore of a greater wall thickness than thesection of drill pipe itself. The reason for this, of course, isapparent inasmuch as it is the box and pin joint to which normally isapplied the slip and other tools for tightening and loosening the joint.

Cathodic protection is obtained for the drill pipe by placing thereon aring member which is made up of metallic material having an electrodepotential more negative than the material of the tubular drill pipesection 20 upon which it is placed. Shown in FIGURE 1 is an upper anodicring 28A and a lower anodic ring 283.

Rings 2 8A and 28B are in intimate contact with the exterior of drillstring section 20. It is preferred that the rings be cast about the pipesection which can be done without affecting the strength of the pipe.The molten zinc alloys with the steel surface, and as the solid zinccools it shrinks more than the steel, all of which assures intimate andpermanent contact between the two metals. The external or outsidediameter of the rings should be less than the external diameter of boxmember 24. The ring members should also be placed as close to the tooljoint 21 as possible, so long as the rings do not interfere with thetools used for making and breaking the joints. F or example, in atypical 4 /2 inch drill pipe the zinc rin-g should be about 8 feet fromthe open end of the box joint on the section of pipe and about 3 feetfrom the open end of the pin joint. While the ring members 28A and 28Bshould have a greater relative electromotive activity than the tubularsection 20, it is preferred that the ring member be zinc when the drillpipe is steel. However, the invention is not limited to Zinc orzinc-rich alloys. Other materials such as magnesium and aluminum alloysmay be more suitable than zinc in some applications.

It is preferred that there be a zinc ring 28A and 28B at each end of thesection 20. While the zinc is functioning as a galvanic anode, it isslowly consumed so the amount of zinc and the longitudinal length of thezinc ring should be great enough to last for the expected life of thedrill pipe. However, it should not greatly increase the weight of thepipe, or have a diameter greater than the tool joints.

Corrosion, as it occurs on drill pipe is an electrochemical process andis associated with the flow of galvanic electric currents. The pipecorrodes at the anodic areas where the current flows from the steel intothe surrounding mud or drilling fiuid. When a more active or more anodicor more negative metal, as zinc, is in contact with the steel pipe, thezinc becomes active or anodic and it corrodes. Thus, the corrosion istransferred from the pipe to the zinc anode. This process is one form ofcathodic protection.

In order to determine an indication of whether the placing of a zincanode around drill pipe is effective, numerous laboratory fatigue testswere made. The machine employed in these tests was similar to theso-cal'led Kenyon Fatigue Test Machine, described in Rotating Wire ArcFatigue Machine for Testing Small Diameter Wire, Proc. A.S.T.M., vol.35, Part II, p. 156 (1935). In the machine used, there were twobearings, one Babbit metal bearing inclined from the vertical, and theother bearing oppositely or counter inclined at a similar angle on thechuck of an electric motor. The wire test specimen was placed in thesebearings and was thus held in curved form and in a vertical plane. Thetest specimens selected were steel wire with carbon content, heattreatment and physical properties similar to normalized drill pipe. Theparticular traction steel wire had a carbon content of .03 to .04% andhad a tensile strength of about 200,000 p.s.i. One end of the wire wasattached to a small electric synchronous motor and the other end wasfree to adjust itself longitudinally in the inclined bearing. Means wereprovided to count the revolutions of the motor. In that machine, as thespecimen was rotated, it automatically eliminated fiexural shear at thefree end and the specimen assumed the form of a circular arc.

The stress-strain relations for the rotating wire specimen are readilyobtained by substituting for the extreme strain which is r/R in therelation:

Stress Strain where E is the elastic modulus of the wire is the radiusof the wire R is the radius of the circle in which the wire is bentafter insertion in the inclined bearing and the chuck on the shaft ofthe synchronous motor.

protected specimen was about 2.5 10

Then the extreme stress is:

Er F

If C is the distance between the inclined bearing and the chuck, and isthe angle of the inclined bearing from the horizontal, then C fi-SIHB(15 Then the stress is:

Ed sine 6 where d is the diameter of the wire.

The wire specimen being tested was placed in the machine and in asolution of 3% NaCl. The wire specimens, when tested in the machine,each had a calculated stress of 40,000 p.s.i., thereon. The stress wascalculated from the above formula. Test A was run on one wire specimenin which there was no cathodic protection. The number of cycles requiredto produce failure in this non-cathodic In another wire specimen in TestB, a zinc ring was molded about the wire in a position so that the ringwas likewise submerged in the fluid or solution of 3% NaCl. Except forthe zinc ring, Test B was identical to Test A. In Test B, the wire didnot break until 102x10 cycles had been obtained. This was a 40-foldincrease in life over that of the specimen in Test A which had nocathodic protection. Even then the wire in Test B broke in the air abovethe solution where cathodic protection was not effective. In the testwith the small zinc ring cast on the wire, the potential was lowered to--.98 volt almost immediately. These data, which are typical, andnumerous other similar tests not herein reported, stringly suggest thatzinc rings cast on drill pipe .are effective in preventing externalcorrosion fatigue of the drill pipe.

While there are disclosed above but a limited number of embodiments ofthe system of the invention herein presented, it is possible to producestill other embodiments without departing from the inventive conceptherein disclosed. It is therefore desired that only such limitations beimposed on the appended claims as are stated therein.

What is claimed is:

1. An apparatus for use in the drilling of a borehole in the earth whichcomprises in combination:

a metallic tubular member;

a material in intimate secure contact with said tubular member, suchmaterial being metallic and having an electrode potential more negativethan the material of the tubular member; and

a drill bit secured at the lower end of said tubular memher.

2. A drill string for use with a bit for drilling a borehole in theearth which comprises:

a metallic tubular member, a first portion of said tubular member beingof a greater diameter than the remaining second portion of the tubularmember;

a zinc ring member surrounding the second portion of said tubular memberin intimate and permanent contact, the outside diameter of said ringbeing not greater than the diameter of said first portion of saidtubular member.

3. An apparatus as defined in claim 2 in which said zinc ring member hasbeen cast around the exterior portion of said second portion of saidtubular member.

4. An apparatus for use with a bit in drilling a borehole in the earthwhich comprises:

a metallic tubular member for supporting said bit within the borehole;and,

a ring member surrounding said tubular member in intimate contacttherewith, said ring member having a greater relative electromotiveactivity than said tubular member.

5. An apparatus for use in drilling a borehole in the v earth whichcomprises:

a metallic tubular member; a material in intimate secure contact withthe exterior of said tubular member, such material being metallic andhaving an electrode potential more negative than that of the tubularmember;

a drill bit secured to the lower end of said tubular memher; and

means secured to said tubular member for protecting said material fromexcessive abrasion by the borehole wall.

6. An apparatus as defined in claim 5 in which said material is cast-onthe exterior of said tubular member.

References Cited by the Examiner UNITED STATES PATENTS 1,646,735 10/1927 Mills 204-448 1,646,736 10/ 1927- Mills 204l97 2,157,180 5/1939Little 204-197 2,735,163 2/1956 Brooks et a1 20 4-497 X JACOB L.NACKENOFF, Primary Examiner.

BENJAMIN BENDETT, Examiner.

W. J. MALONEY, Assistant Examiner.

4. AN APPARATUS FOR USE WITH A BIT IN DRILLING A BOREHOLE IN THE EARTHWHICH COMPRISES: A METALLIC TUBULAR MEMBER FOR SUPPORTING SAID BITWITHIN THE BOREHOLE; AND,